In version 7.6.1, Plugins Admin is enhanced on several aspects. However, the enhancement could bring some plugins issues for people who update their Notepad++, due to the new location for plugins. Sorry for the inconvenient.

In order to remedy this problem, after the update, if you update from 7.6 :
Move all %LOCALAPPDATA%\Notepad++\plugins\ (except “Config” folder) to %PROGRAMDATA%\Notepad++\plugins.

Otherwise, you update from v7.5.9 or version older than v7.5.9:
Find the name of plugin you want to migrate in %PROGRAMFILES(x86)%\Notepad++\plugins\myAwesomePlugin.dll (or %PROGRAMFILES%\Notepad++\plugins\myAwesomePlugin.dll, according the achetecture of your Notepad++).
go to %PROGRAMDATA%\Notepad++\plugins\ to create myAwesomePlugin folder.
move the plungin myAwesomePlugin.dll from %PROGRAMFILES(x86)%\Notepad++\plugins\ to %PROGRAMDATA%\Notepad++\plugins\myAwesomePlugin.

Just tested it, if you treat the former plugin manager in the same way
as the new plugins, meaning, creating a folder PluginManager under
plugins directory and copy PluginManager.dll into it, it is available.

But, obviously doesn’t work as expected in terms of installing plugins.

correct, how should it know about the new folder structure.
One thing which might be useful is to use it to download the plugins
which aren’t migrated yet. Create the directory, copy the downloaded plugin - done - hopefully.

Since version 7.6, the ANSI code page is sometimes incorrectly detected. A file containing text “CHRÁNĚNÉ”, encoded in ANSI (Windows with the Czech Republic region selected, i.e. the Windows 1250 code page), is opened as “CHRÁNÌNÉ” (in both versions 7.6 and 7.6.1) with the automatically selected code page ISO 8859-3.

For file with more Czech letters (e.g. “ěščřžýáíé” at the first row) is ANSI code page selected properly, but text “CHRÁNĚNÉ” was also enough to select ANSI in 7.5.9 (and the older ones).

Some ANSI files (with Czech letters) are incorrectly opened in ISO 8859-2.

So far I have to stay on version 7.5.9, in which the ANSI code page is always selected correctly.

haven’t had seen this before but could you check what happens if
you uncheck enable notepad++ auto-updater fromsettings->preferences->misc dialog?
Maybe restart npp afterwards and try to install plugins.
If this doesn’t help would you mind posting the debug-info from ? menu?